Conventional holographic digital data storage systems normally employ a page-oriented storage approach. An input device such as an SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page). Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-oriented approach. All of these systems, however, suffer from a common drawback in that they require the recording of a huge number of separate holograms in order to fill the memory to capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system such as the one shown in FIG. 1A has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
The conventional holographic storage system (see “Holographic disk recording system”, U.S. patent application publication No. US2003/0161246A1, by Ernest Chuang, et al.) shown in FIG. 1A includes a light source 100; HWPs (half wave plates) 102, 112; an expanding unit 104; a PBS (polarization beam splitter) 106; polarizers 108, 114; mirrors 110, 116, 117; a mask 122; a holographic medium 120; and a conical prism 118.
The light source 100 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam of only one type of linear polarization, e.g., either P- or S-polarization, is provided to the HWP 102. The HWP 102 rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is fed to the expanding unit 104 for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS 106.
The PBS 106, which is manufactured by repeatedly depositing at least two kinds of materials, each having a different refractive index, serves to transmit one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of polarized laser beam, e.g., S-polarized beam. Thus the PBS 106 divides the expanded laser beam into a transmitted laser beam (hereinafter called a signal beam) and a reflected laser beam (hereinafter called a reference beam) having different polarizations, respectively.
The signal beam, e.g., of a P-polarization, is fed to the polarizer 108, which removes imperfectly polarized components of the signal beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the signal beam with perfect or purified polarization is reflected by the mirror 110. Thereafter, the reflected signal beam is projected onto the holographic medium 120 via the mask 122. The mask 122, presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).
Meanwhile, the reference beam is fed to the HWP 112. The HWP 112 converts the polarization of the reference beam such that the polarization of the reference beam becomes identical to that of the signal beam. And then the reference beam with converted polarization is provided to the polarizer 114, wherein the polarization of the reference beam is further purified. And the reference beam with perfect polarization is reflected by the mirror 116 and then the mirror 117 sequentially. Thereafter, the reflected reference beam is projected onto the conical prism 118 (the conical prism 118 being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The reflected reference beam is refracted toward the holographic medium 120 by the conical prism 118. The angle of incidence of the refracted reference beam on the holographic medium 120 is determined by the base angle of the conical prism 118.
The holographic medium 120 is preferably of a disk-shaped material for recording the data patterns. The mask 122, also having a disk shape of a similar size to that of the holographic medium 120, provides the data patterns to be stored in the holographic medium 120. By illuminating the mask 122 with a normally incident plane wave, i.e., the signal beam, and by using the reference beam incident from the opposite side to record holograms in the refraction geometry, the diffracted pattern is recorded in the holographic medium 120. Furthermore, an angular multiplexing can be realized by using the conical prism 118 with a different base angle.
FIG. 1B depicts optical paths of the reference beam from the conical prism 118 to the holographic medium 120 in the conventional holographic storage system of FIG. 1A.
The circular base of the conical prism 118 is preferably parallel with the holographic medium 120, and does not face the holographic medium 120, i.e., a vertex of the conical prism 118 faces the holographic medium 120. The holographic medium 120 is provided with a hole region 120b at the center thereof and an annular-shaped recording region 120a therearound. Further, the symmetry axis of the holographic medium 120 is coincident with that of the conical prism 118 passing through the vertex thereof.
As shown in FIG. 1B, the reference beam with a radius of W1 strikes the circular base of the conical prism 118. The reference beam propagates in a first propagating direction which is perpendicular to the holographic medium 120, i.e., to the circular base. The reference beam is not refracted at the circular base, because the first propagating direction is normal to the circular base. Thus, the reference beam also propagates in the medium of the conical prism 118 in the first propagating direction until the reference beam reaches the surface of the cone of the conical prism 118. At the surface of the cone, the reference beam is refracted, to thereby produce the refracted reference beam which then propagates toward the holographic medium 120 in the medium of the air in a second propagating direction as shown in FIG. 1B, while obeying Snell's law.
In FIG. 1B, the surface of the cone includes a first half cone surface 118a and a second half cone surface 118b. Moreover, the holographic medium 120 includes a first half recording region 120aa which is located on the same side as that of the first half cone surface 118a and a second half recording region 120ab which is located on the same side as that of the second half cone surface 118b. A first half reference beam is transmitted to the first half cone surface 118a after passing through the circular base and then refracted toward the second half recording region 120ab and a second half reference beam is transmitted to the second half cone surface 118b after passing through the circular base and then refracted toward the first half recording region 120aa. 
Therefore, as shown in FIG. 1B, near the holographic medium 120, the refracted first half reference beam and the refracted second half reference beam form a conical shell beam shape whose center portion is empty so that a cross section of the conical shell beam shape cut by a plane parallel with the holographic medium 120 may be an annular shape.
After the reference beam with a radius of W1 is refracted at the cone surface 118, the size of the refracted reference beam (i.e., the width thereof being equal to one-half of the difference between the outer and the inner diameters of the annular-shaped cross section thereof) is decreased down to W2 because a refractive index of the conical prism 118 is larger than 1.
In case the size of the recording region 120a (i.e., the width thereof being equal to one-half of the difference between the outer and the inner diameters thereof) of the holographic medium 120 is W3 as shown in FIG. 1B, the size W2 of the refracted reference beam needs to be equal to or larger than W3 in order to write data on the recording region at once.
However, there is a critical problem in the prior art as follows.
A radius of the circular base of the conical prism 118 is preferably slightly larger than (approximately equal to) that of the reference beam W1 such that the entire refracted reference beam with the size of W2 is irradiated onto the recording region 120a with the size of W3. However, since the radius of the circular base is larger than W2 which is equal to or larger than W3, the size of the conical prism 118, i.e., the radius of the circular base, should be larger than that of the recording region 120a. Thus, the size of the conical prism 118 becomes larger so that the conventional holographic digital data storage system cannot be miniaturized.